Prescalers are used in the feedback loop between the output of a voltage-controlled oscillator (VCO) and the phase frequency detector in phase locked loop frequency synthesizers to generate higher frequencies. A multi-modulus prescaler is one in which the division ratio can be switched among the multiple division ratios by an external control signal to extend the upper frequency range of the synthesizer but still allow the synthesis of lower frequencies. For example, a four-modulus prescaler offers four different scaling factors and two control signals are required to select one of the four different scaling factors.
A disadvantage of multi-modulus prescalers is that unwanted spurious frequencies are generated by the operation of the multi-modulus. These unwanted spurious frequencies are generally located around the desired frequency, which adds noise to the loop. The generation of spurious frequencies is undesirable; particularly when the phase locked loop frequency synthesizer is used as a local oscillator in a communication system, particularly multi-user, radio communication systems such as, for example, cellular communication systems.
Another disadvantage of multi-modulus prescalers is the limitation on speed at the system level that results from a divider with a changing division ratio, which is slower when compared to the speed of a system with a fixed division ratio. The difference in speed is due to the additional logic inserted in the loop to select the divider ratio which introduces longer delays into the loop. It is known to select the division ratio outside the loop by using the phase in quadrature from the divider to accelerate the system. A multiplexer (MUX) located after the divider places the proper phase at the output. Although selection of the division ratio in this manner may speed up the system operation, the problem of spurious frequency generation is still present because the MUX does not always return to its original phase.
It would be desirable therefore to provide a fractional multi-modulus prescaler that overcomes the problems generally associated with known prescalers.
It is an object of the present invention therefore to provide a fractional multi-modulus prescaler without modulus caused spurious frequencies.